1. Field of the Invention
This invention relates generally to marine seismic surveying, and, more particularly, to a coupling aid for seismic cables used in marine seismic surveying.
2. Description of the Related Art
Seismic exploration is widely used to locate and/or survey subterranean geological formations for hydrocarbon deposits. As many commercially valuable hydrocarbon deposits are located beneath bodies of water, various types of marine seismic surveys have been developed. In a typical marine seismic survey, an array of marine seismic cables is deployed at the sea-floor. The seismic cables may be several thousand meters long and contain a large number of seismic sensors, such as hydrophones and geophones, associated electronic equipment, which are distributed along the length of the each seismic cable. Power is provided to the seismic sensors via wires, cables, and the like, which are run through the seismic cable. Signals may also be transmitted to and from the seismic sensors by additional wires, cables, and the like.
The seismic sensors typically include one or more seismic sensing elements deployed in a sensor housing. The sensor housing protects the seismic sensing elements from being exposed to external pressure and from water intrusion when they are deployed underwater. For example, the sensor housing may reduce the amount of sea-water that reaches the seismic sensing elements and therefore may reduce the corrosive effects of the sea water on the seismic sensing elements. When the sensor is disposed on the sea floor, the sensor housing couples the seismic sensing elements to the sea-floor. The seismic coupling allows signals, e.g. acoustic signals, travelling in the sea-floor to be sensed and recorded by the seismic sensing elements in the sensor housing.
A survey vessel typically tows one or more seismic sources, such as airguns and the like, over the seismic cables. Acoustic signals, or “shots,” produced by the seismic sources propagate down through the water into the earth beneath, where they are reflected from the various earth strata. The reflected signals are received by the seismic sensors in the seismic cables, digitised, and then transmitted to the seismic survey vessel, where they are recorded and at least partially processed with the ultimate aim of building up a representation of the earth strata in the area being surveyed. Analysis of the representation may indicate probable locations of geological formations and hydrocarbon deposits.
The reflected signals include a pressure and an elastic wave field. The pressure is a scalar quantity and the elastic wave-field is a vector quantity. Thus, to characterise the elastic wave-field, the seismic sensing elements measure the components of the elastic wave-field in three orthogonal directions. For example, the selected three directions may be an x-direction, which is defined as being parallel to the seismic cable and is often referred to as the “in-line direction,” a y-direction, which is defined as being perpendicular to the seismic cable and is often referred to as the “cross-line direction,” and a z-direction, which is defined as the vertical direction. The use of this so-called “four-component” seismic data has proven to be very successful for imaging through gas-saturated overburdens and for characterising hydrocarbon reservoirs through lithography identification and fluid discrimination.
The reliability of conventional methods of recording four-component seismic data is typically reduced by vector infidelity between the in-line and the cross-line directions. For example, FIG. 1A shows the x-component 100 and y-component 110 of the elastic wave-field as measured by a conventional seismic sensor 120, as shown in FIG. 1B. The seismic energy 125 in the elastic wave-field provided by a seismic source 130 is incident on the seismic sensor 120 at a 45° angle. Since the seismic energy 125 makes an equal angle with the x-direction and the y-direction, the amplitudes of the x-component 100 and y-component 110 of the elastic wave-field should be equal.
However, poor seismic coupling between the sensor housing and the seafloor provided by conventional sensor housings may create vector infidelities. These vector infidelities may result in the seismic sensor recording different amplitudes for the x-component 100 and y-component 110 of the elastic wave-field, as shown in FIG. 1A. For example, cylindrical sensor housings may roll on the sea floor, which may reduce the fidelity of recordings of cross-line components and may also reduce the fidelity of the vertical component.
A variety of hardware solutions have been developed to attempt to reduce vector infidelity in data acquisition. For example, the patent U.S. Pat. No. 6,288,972 entitled, “Cable Mounted Cleated Housing for Engaging the Seabed,” describes cleats that may be mounted on the outside of the sensor housing. The cleated boots may improve the seismic coupling by penetrating the sediments on the sea-floor. However, the cleats sometimes may not allow good transfer of motion perpendicular to the cable and sometimes may not be able to avoid mechanical resonances. The cleated boot may also not be able to rotate and may therefore be deployed on an edge, which may further increase the vector infidelity. For another example, the patent number WO 02/14905 A1, entitled “A Housing for a Seismic Sensing Element and a Seismic Sensor,” describes a seismic sensor housing with a substantially flat base wherein the maximum extent of the base in the in-line and the cross-line directions is substantially equal. However, this seismic sensor housing is cumbersome and difficult to handle.
Deconvolution operators may be designed to partially reduce vector infidelity in the analysis of collected data. For example, the patent publication WO 01/51955 discloses a method of correcting seismic data for vector infidelity by generating a correction factor. For another example, an alternative deconvolution method has been proposed in the publication, “Compensating OBC data for variations in geophone coupling,” Proceedings of the 68th Annual Meeting of the Society of Exploration Geophysicists (1998), pp. 1429-1432. However, the distortions of the components of the elastic wave-field are often too large for deconvolution to remove the vector infidelity during analysis.